CONCEPTS IN BIOLOGY

PART VI. PHYSIOLOGICAL PROCESSES

26. The Body's Control Mechanisms and Immunity

26.3. The Endocrine System

In the 1890s, many physicians began to describe the workings of chemicals in the body, suggesting that they were “internal secretions.” Ernest Starling named these chemical messengers hormones. A hormone is a specific molecule that is released by one organ and transported to another organ, where it triggers a change in the other organ’s activity. The endocrine system consists of a number of glands that communicate with one another and with other tissues through chemicals distributed throughout the organism. Glands are organs that manufacture molecules that either are secreted into surrounding tissue, where they are picked up by the circulatory system, or are secreted through ducts into the cavity of an organ or to the body surface. Endocrine glands have no ducts; they secrete their products—hormones—into the circulatory system (figure 26.9). Other glands, such as the digestive glands and sweat glands, empty their contents through ducts. These kinds of glands are called exocrine glands.

FIGURE 26.9. Endocrine Glands

The endocrine glands, located in various places in the body, secrete hormones.

Endocrine System Function

As with the nervous system, it is helpful to have a general idea about how the endocrine system works as it is discussed.

The endocrine system functions the way a radio broadcast system does. Radio stations send their signals in all directions, but only the radio receivers that are tuned to the correct frequency can receive the signals. Messenger molecules (hormones) are distributed throughout the body by the circulatory system, but only the cells that have the proper receptor sites can receive and respond to them. The cells that are able to respond to a hormone are called target cells; they respond in one of three ways: (1) Some cells release products that have been previously manufactured; (2) some cells synthesize molecules or begin metabolic activities; and (3) some cells divide and grow. As a result of these different kinds of responses, some endocrine responses are relatively rapid, whereas others are very slow. For example, the release of the hormones epinephrine and norepinephrine (formerly called adrenalin and noradrenalin) from the adrenal medulla, located near the kidney, causes a rapid change in an organism’s behavior. The heart rate increases, blood pressure rises, blood is shunted to the muscles, and the breathing rate increases. You have certainly experienced this reaction many times in your lifetime, such as when you nearly had an automobile accident or slipped and nearly fell.

Antidiuretic hormone (ADH) acts more slowly. It is released from the posterior pituitary gland at the base of the brain; it regulates the rate at which the body loses water through the kidneys. It does this by encouraging the reabsorption of water from the collecting ducts of the kidneys (see chapter 24). The effects of ADH can be noticed in a matter of minutes to hours.

Insulin is another hormone whose effects occur within minutes. It is produced by the pancreas, located near the stomach. Insulin stimulates cells—particularly muscle, liver, and fat cells—to take up glucose from the blood. After a high-carbohydrate meal, the glucose level in the blood begins to rise, stimulating the pancreas to release insulin. The increased insulin causes glucose levels to fall as the sugar is taken up by the cells. People with diabetes have insufficient or improperly acting insulin or lack the receptors to respond to the insulin; therefore, they have difficulty regulating glucose levels in their blood.

The responses that result from the growth of cells may take weeks or years to occur. For example, growth- stimulating hormone (GSH) is produced by the anterior pituitary gland over a period of years and results in typical human growth. After sexual maturity, the amount of this hormone generally drops to very low levels, and body growth stops. Sexual development is also largely the result of the growth of specific tissues and organs. The male sex hormone testosterone, produced by the testes, causes the growth of male sex organs and a change to the adult body form. The female counterpart, estrogen, results in the development of female sex organs and body form. In all of these cases, it is the release of hormones over long periods, continually stimulating the growth of sensitive tissues, that results in a normal developmental pattern. The absence or inhibition of any of these hormones early in life changes the normal growth process.

Negative-Feedback Inhibition and Hormones

Many endocrine glands and their hormones are under negative-feedback inhibition. At the beginning of the chapter, insulin and glucagon were used as examples of molecules involved in simple negative-feedback inhibition. However, glands within the endocrine system often interact with one another so that the secretions of one gland alter the actions of others. When several glands interact in negative-feedback inhibition, an increased amount of one hormone interferes with the production of a different hormone in the chain of events. The production of thyroxine and triiodothyronine by the thyroid gland is regulated by negative-feedback inhibition involving the actions of other glands. The production of these two hormones is stimulated by the increased production of a hormone from the hypothalamus called thyroidreleasing hormone (TRH) which stimulates the anterior pituitary to produce a hormone called thyroid-stimulating hormone (TSH). The thyroid gland’s control lies in the quantity of TSH produced. When the anterior pituitary produces high levels of thyroid-stimulating hormone, the thyroid is stimulated to grow and secrete more thyroxine and triiodothyronine. But when increased amounts of thyroxine and triiodothyronine are produced, these hormones have a negative effect on the pituitary and hypothalamus, so that production of thyroid-stimulating hormone and thyroidreleasing hormone are decreased, leading to reduced production of thyroxine and triiodothyronine. If the amount of the thyroid hormones falls too low, the pituitary and hypothalamus are no longer inhibited and begin to release their hormones. As a result of the interaction of these hormones, their concentrations are maintained within certain limits (figure 26.10).

FIGURE 26.10. Negative-Feedback Inhibition of Thyroid Secretion

The hypothalamus sends the thyroid-releasing hormone (TRH) to the pituitary, which releases thyroid-stimulating hormone (TSH). Thyroid-stimulating hormone causes the thyroid to produce thyroxine and triiodothyronine. These two hormones inhibit the hypothalamus and pituitary.

26.3. CONCEPT REVIEW

6. How do exocrine and endocrine glands differ?

7. Give an example of negative-feedback control in the endocrine system.